A vote might be close: opinion polls suggest Britons aren’t convinced they want to stay part of the 28-nation bloc, at least in its current form.

Lawmakers aren’t entirely convinced either. Prime Minister David Cameron — who has promised the referendum if his Conservative Party is still in power after 2015 elections — says he wants to stay in, but only if there is significant reform and the U.K. is given back some powers to set its own laws. Neither of those is a small ask. (Germany’s premier Angela Merkel, in the U.K. Thursday, said some reform of the EU may be possible — but the U.K. should commit to staying a member first.)

All of this is in stark contrast to the point of view of businesses. Company leaders are horrified by the idea of a U.K. exit.

A poll of companies last year by the Confederation of British Industry, a business lobby, showed almost 80% want the country to stay in the bloc. The same proportion of German firms present in the U.K. say EU membership is important or very important to their operations. The chief executives of Ford Motor Co. and Nissan Motor Co. Ltd have warned that future investment in Britain could hinge on it staying in the bloc.

Why do businesses have such a different viewpoint? One reason may be that while voters fret about issues such as high levels of immigration — often blamed on EU laws — businesses are more concerned by the economics.

“Membership of the single market is the crucial aspect,” Peter Fouquet, Chairman of the German-British Chamber of Industry and Commerce, said in an interview earlier this month with the Wall Street Journal. “There are a number of important benefits of a single market, including the ability to make investment decisions quickly, the ease of sourcing and selling goods, and the ability to recruit and deploy staff without restrictions.”

“If Britain were to leave the EU and the single market, it could find itself out of the global supply chains, and in the medium term face the prospect of receiving less investment from companies favoring the EU as a destination,” said Mr. Fouquet, who also heads of U.K. operations of German equipment maker Robert Bosch GmbH.

The single market ensures free cross-border movement of goods, services, capital and people between the 28 EU nations.

Some countries that aren’t in the EU, such as Norway, Liechtenstein and Iceland, are allowed to operate in the single market, having struck a joint deal that requires them to comply with EU rules.

A U.K. outside the EU would have to forge trade agreements with the remaining member states to keep using the single market. The uncertain outcome of such talks is what worries businesses.

If the conservatives lose next year’s election, there may well not be a referendum: the main opposition Labour party has a more pro-EU stance and hasn’t committed to holding a vote.

But if there is a referendum, it seems unlikely that ordinary voters will be swayed purely by arcane economic issues such as the single market—no matter how crucial businesses think it is.

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